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  • Profiles of near-surface rock mass strength across gradients in erosion, burial, and time [Data set]

    work
    Creator:
    Townsend, Kirk F, Clark, Marin K, and Zekkos, Dimitrios
    Description:
    These datasets support the findings of Townsend et al. (2020). In this article, we project profiles of rock mass shear strength into the shallow subsurface (~30 m depth) using the Hoek and Brown criterion with Geological Strength Index (GSI) observations of outcrop structure and surface conditions, and Schmidt hammer rebound values of intact (unfractured) rock hardness. We compare these projected rock mass shear strength profiles to shear-wave velocity profiles collected using shallow geophysical arrays. We evaluate our methods in the Western Transverse Ranges of southern California, which exhibit strong gradients in the depth of latest-Mesozoic through Cenozoic sedimentary rocks exposed at the surface today, and in erosion rates quantified from catchment-average cosmogenic radionuclide concentrations and low-temperature apatite and zircon (U-Th)/He thermochronometry. We find that stratigraphic age and burial depth exerts the strongest apparent control on rock strength and S-wave velocities, likely due to diagenetic changes associated with burial. For rocks of the same age and inferred burial history, we observe that shear strength and S-wave velocities are positively correlated with erosion rate. We suggest that increasing erosion rates cause decreased residence time of rock masses within the critical zone, resulting in less weathered rocks.
    Keyword:
    rock strength, seismic, and erosion rate
    Citation to related publication:
    Townsend, K. F., Clark, M. K., & Zekkos, D. (2021). Profiles of Near-Surface Rock Mass Strength Across Gradients in Burial, Erosion, and Time. Journal of Geophysical Research: Earth Surface, 126(4), e2020JF005694.  https://doi.org/10.1029/2020JF005694
    Discipline:
    Science

  • Geotechnical observations of weathered rock across a tectonic and climatic gradient in Central Nepal

    work
    Creator:
    Medwedeff, William, G (University of Michigan Earth & Environmental Science), Clark, Marin, K (University of Michigan Earth & Environmental Science), Zekkos, Dimitrios (University of California, Berkeley), West, A., Joshua (University of Southern California), and Chamlagain, Deepak (Tribhuvan University, Kathmandu Nepal)
    Description:
    These datasets support the findings of Medwedeff et al. (2021) in JGR: Earth Surface. In this article, we present seismic and geotechnical characterizations of the shallow subsurface across a 200 km by 50 km swath of the central Himalayan Range, in Nepal. By pairing widely-distributed 1D shear wave velocity surveys and engineering outcrop descriptions per the Geological Strength Index classification system, we evaluate landscape-scale patterns in near-surface mechanical characteristics and their relation to environmental factors known to affect rock strength. We find that near-surface strength is more dependent on the degree of weathering, rather than the mineral and textural differences between the metamorphic lithologies found in the central Himalaya. Furthermore, weathering varies systematically with topography. Bedrock ridge top sites are highly weathered and have S-wave seismic velocities and shear strength characteristics that are more typical of engineering soils, whereas sites near the bedrock channel bottom tend to be less weathered and characterized by high S-wave velocities and shear strength estimates typical of hard rock. Weathering of bedrock on hillslopes is significantly more variable, resulting in S-wave velocities that range between the ridge and channel endmembers. We hypothesize variability in the hillslope environment may be partly explained by the stochastic nature of mass wasting, which clears away weathered material where landslide scars are recent. These results underscore the mechanical heterogeneity in the shallow subsurface and highlight the need to account for bedrock weathering when estimating strength parameters for regional landslide hazard analysis.
    Keyword:
    rock strength, critical zone, shallow seismic, and chemical weathering
    Discipline:
    Science